LiCoPO4 Cathode Research Articles

Effect of Fe and Zn co-doping on LiCoPO4 cathode materials for High-Voltage Lithium-Ion batteries

Lithium cobalt phosphate (LiCoPO4) has great potential to be developed as a cathode material for lithium-ion batteries (LIBs) due to its structural stability and higher voltage platform with a high theoretical energy density. However, the relatively low diffusion of lithium ions still needs to be improved. In this work, Fe and Zn co-doped LiCoPO4: LiCo0.9-xFe0.1ZnxPO4/C is utilized to enhance the battery performance of LiCoPO4. The electrochemical properties of LiCo0.85Fe0.1Zn0.05PO4/C demonstrated an initial capacity of 118 mAh/g, with 93.4 % capacity retention at 1C after 100 cycles, and a good capacity of 87 mAh/g remained under a high current density of 10C. In addition, the diffusion rate of Li ions was investigated, proving the improvement of the materials with doping. The impedance results also showed a smaller resistance of the doped materials. Furthermore, operando X-ray diffraction displayed a good reversibility of the structural transformation, corresponding to cycling stability. This work provided studies of both the electrochemical properties and structural transformation of Fe and Zn co-doped LiCoPO4, which showed that 10 % Fe and 5 % Zn co-doping enhanced the electrochemical performance of LiCoPO4 as a cathode material in LIBs.

One-Step Synthesis of LiCo1-1.5xYxPO4@C Cathode Material for High-Energy Lithium-ion Batteries.

Intrinsically low ion conductivity and unstable cathode electrolyte interface are two important factors affecting the performances of LiCoPO4 cathode material. Herein, a series of LiCo1-1.5xYxPO4@C (x = 0, 0.01, 0.02, 0.03) cathode material is synthesized by a one-step method. The influence of Y substitution amount is optimized and discussed. The structure and morphology of LiCo1-1.5xYxPO4@C cathode material does not lead to obvious changes with Y substitution. However, the Li/Co antisite defect is minimized and the ionic and electronic conductivities of LiCo1-1.5xYxPO4@C cathode material are enhanced by Y substitution. The LiCo0.97Y0.02PO4@C cathode delivers a discharge capacity of 148 mAh g−1 at 0.1 C and 96 mAh g−1 at 1 C, with a capacity retention of 75% after 80 cycles at 0.1 C. Its good electrochemical performances are attributed to the following factors. (1) The uniform 5 nm carbon layer stabilizes the interface and suppresses the side reactions with the electrolyte. (2) With Y substitution, the Li/Co antisite defect is decreased and the electronic and ionic conductivity are also improved. In conclusion, our work reveals the effects of aliovalent substitution and carbon coating in LiCo1-1.5xYxPO4@C electrodes to improve their electrochemical performances, and provides a method for the further development of high voltage cathode material for high-energy lithium-ion batteries.

Wet milling synthesis of ammonium cobalt phosphate hydrate platelets for LCP‐olivine cathodes of LIBs using a bead mill

AbstractIn this study, NH4CoPO4·H2O (ACP) platelets were synthesized from NH4H2PO4 and Co(OH)2 by wet milling process using a bead mill. ACP is one of the precursors of LiCoPO4 (LCP) cathode materials for Li‐ion batteries. In the wet milling, ACP large platelets were synthesized in a short time at the beginning, and the large platelets were gradually cleaved into the smaller platelets. The lateral sizes of the ACP platelets were several 10 μm at 30 min and about 10 μm at 5 h. It was found that the ACP platelets were synthesized efficiently by using the bead mill as a promising machine for the scale‐up in manufacturing. The ACP platelets were converted into LCP by mixing with Li2CO3 and heating, whereas platelet shapes were retained. The initial discharge capacities of the cathodes using the LCP platelets converted from the ACP of 30 min and 5 h milling were 70 and 95 mAh/g at 0.05 C, respectively. It is because the specific surface area of the latter sample (3.3 m2/g) was larger than that of the former (2.2 m2/g). The larger specific surface area possibly led to the increase of contacts with electron conductive additives (carbon) and electrolytes.

Preliminary Study of LiCoPO4 cathode material synthesis for LiCoPO4/Graphite Cell

<p>Nowadays, the race towards a high-performance Li-ion batteries discovery for energy storage has become an interesting technological challenge. New or improved Li-ion battery cathodes have been developed relentlessly in this decade. LiCoPO<sub>4</sub> (LCP) is a cathode material that allow a battery to operate at high voltage which resulted a high energy density battery. The presence of strong P-O bond adds a thermal stability feature to LIB cells. In this study, a LiCoPO<sub>4</sub> material were prepared by precipitation of Co<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> (CP) followed by high temperature Li insertion. H­<sub>3</sub>PO<sub>4</sub> were used as the phosphate source. The crystallization or precipitation of CP can only be achieved under mild pH condition, which can be controlled using sodium hydroxide. XRD, SEM and FTIR analysis were used to characterize the final product. The as-obtained LCP were applied as cathode material while graphite was used as the counter anode. The cell can deliver a charge-discharge capacity at a voltage of 2.0-4.95 V. This preliminary can be a groundwork toward the commercialization of LCP based Li-ion batteries for high duty application.</p>

Stabilizing the high voltage LiCoPO4 cathode via Fe-doping in the gram-scale synthesis

We report the effect of Fe doping on the electrochemical performance of the olivine structure LiCoPO4. Here, we employed a solid-state approach followed by a carbon coating for the synthesis of LiFexCo1-xPO4@C (0 < x < 0.25) in a simple two-step procedure. The structural characteristics of the gram-scale prepared cathodes are investigated by various analytical tools and observed that ∼10 wt.% of carbon content efficiently enhances the electrical conductivity. The galvanostatic studies endorses an enhancement in the electrochemical performance of LiFexCo1-xPO4@C (0 < x < 0.25) compared to bare LiCoPO4. Among the various as-synthesized Fe doped samples, LiFe0.1Co0.9PO4@C and LiFe0.15Co0.85PO4@C perform excellent capacity retention characteristics of 67 and 78% after 50 cycles, respectively, and are optimized to be the better Fe dopant concentrations. Further, the cyclic voltammetry and electrochemical impedance spectroscopy studies display better electronic as well as, Li+ diffusion kinetics upon Fe doping. As a proof of concept, enhancement in Li+ ion chemical diffusion co-efficient is also observed in the range of 10–12 cm2 s–1 in LiFexCo1-xPO4@C (0 < x < 0.25) whereas only 10–15 cm2 s–1 for LiCoPO4. These outcomes recommend this valuable approach toward a gram scale preparation of high voltage Fe-doped LiCoPO4.

Recent advancements in LiCoPO4 cathodes using electrolyte additives

Olivine-type LiCoPO 4 proves its intriguing electrochemical properties as a highly valuable voltage cathode material for the next-generation lithium-ion batteries (LIB) with the benefits of a high operating potential of ∼4.8 V versus Li and a high theoretical capacity of ∼167 mAh g −1 . However, several limitations in attaining high coulombic efficiency, good rate capability, and long cycle stability need to be improved before implementing into commercial applications. Thus, various strategies involve advancing the LiCoPO 4 performances via synthesis routes, surface modification, doping with iso- and alio-valent substitutions, etc., to achieve competitive performance with other commercial cathodes. Apart from these strategies, suitable electrolytes and additives are equally important to alleviate the electrolyte oxidation at high voltages to develop next-generation LIBs. In this work, we summarize the structural and physical properties of LiCoPO 4 with information about the strategies to enhance the performance of LiCoPO 4 in the beginning and discuss the method of electrolyte optimization using various additives in detail. Future perspectives dictate the important insights into practical issues that can accelerate the progress of LiCoPO 4 -based LIBs in energy storage applications toward practical use.

Fabrication of 4.7 V class “rocking-chair” type Li-ion cells with carbon-coated LiCoPO4 as cathode and graphite anode

We report the fabrication of a “rocking-chair” type high-energy Li-ion battery with a high voltage LiCoPO4 cathode with a graphite anode. We adopted scalable solid-state synthesis to yield the LiCoPO4 particles and subsequently carbon-coated by decomposing the glucose. Prior to this process, LiCoPO4 is ball-milled to enable homogeneous carbon-coating. Eliminating irreversible capacity is very important for the fabrication of full-cell, in which we adopted four different strategies. The LiCoPO4-C/graphite cell is capable of working at the potential of ~4.7 V with an energy density of ~326 Wh kg−1.

Wet milling synthesis of NH4CoPO4·H2O platelets: Formation reaction, growth mechanism, and conversion into high-voltage LiCoPO4 cathode for Li-ion batteries

A ball milling process not only affords particle size reduction by grinding but also particle synthesis induced by mechanical actions at ambient temperature. However, in traditional mechanical synthesis, it is difficult to control the particle morphology, including the size and shape, because of collision with the balls. This study shows that platelet-like NH4CoPO4·H2O particles are synthesized via wet planetary ball milling and converted into a high-voltage LiCoPO4 cathode for Li-ion battery. The NH4CoPO4·H2O platelets formed by the dissolution–precipitation reaction are repeatedly ground, dissolved into a solution, and recrystallized to afford crystal growth during the milling treatment. The converted LiCoPO4 cathodes retain the precursor morphologies, exhibiting high discharge capacities for flake particles and better cyclabilities for large platelet particles. The mechanical-assisted particle synthesis in solution is a simple method for the large-scale production of morphology-controlled nanomaterials.

High-Voltage Lithium-Ion Battery Using Substituted LiCoPO4: Electrochemical and Safety Performance of 1.2 Ah Pouch Cell

A LiCoPO4-based high-voltage lithium-ion battery was fabricated in the format of a 1.2 Ah pouch cell that exhibited a highly stable cycle life at a cut-off voltage of 4.9 V. The high-voltage stability was achieved using a Fe-Cr-Si multi-ion-substituted LiCoPO4 cathode and lithium bis(fluorosulfonyl)imide in 1-methyl-1-propylpyrrolidinium bis(fluorosulfony)imide as the electrolyte. Due to the improved electrochemical stability at high voltage, the cell exhibited a stable capacity retention of 91% after 290 cycles without any gas evolution related to electrolyte decomposition at high voltage. In addition to improved cycling stability, the nominal 5 V LiCoPO4 pouch cell also exhibited excellent safety performance during a nail penetration safety test compared with a state-of-the-art lithium ion battery. Meanwhile, the thermal stabilities of the 1.2 Ah pouch cell as well as the delithiated LiCoPO4 were also studied by accelerating rate calorimetry (ARC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and in situ X-ray diffraction (XRD) analyses and reported.

The study on the effect of hydrothermal temperature on the structure and characteristics of olivine LiCoPO4 cathode materials

The experiment was conducted to research and fabricate single phase olivine LiCoPO4 materials with nano-meter structure by hydrothermal method at different temperatures of 190oC, 200oC and 240oC. Methods of determining the structure and characteristics of materials include: XRD, FTIR, SEM. The ion conductivity of the LiCoPO4 powder materials suspension was measured on a conductivity meter (Hana Instruments HI2314). The results showed that, when the hydrothermal temperature increased from 190-240oC, the crystal formation and development gradually increased. The average crystal sizes of the samples were about 23.04 nm, 25.75 nm and 30.19 nm which were gradual increase with hydrothermal temperature. The wave vibrations in the FTIR spectra showed the bounding vibrations of the O-P-O that were of the tetrahedron PO43- and bonding vibrations of metal-O. The ionic conductivity of the olivine LiCoPO4 powder samples suspension increases when the hydrothermal temperature of samples increases of 190, 200 and 240oC. The maximum electrical conductivity was ~39.4 mS/cm obtained by olivine LiCoPO4 sample which was hydrothermal at 200oC.

A high voltage Li-ion full-cell battery with MnCo2O4/LiCoPO4 electrodes

A new full-cell is reported herein that combines a high capacity conversion-type anode and an olivine-type 5 V cathode to develop a high energy Li-ion battery. The MnCo2O4 anode was prepared through a simple solvothermal process followed by a post-annealing treatment. The synthesized MnCo2O4 exhibited characteristic morphology consisting of spherical microspheres constructed from nanoflake units. The olivine-type LiCoPO4 cathode was synthesized by a rapid single-step microwave-assisted solvothermal approach using a 1:1 (v/v) ethylene glycol/water binary solvent mixture and sucrose as the reducing agent. After structural characterization of the electrodes by X-ray diffraction, their electrochemical response characteristics related to the lithium insertion/extraction in half-cells and those of the MnCo2O4//electrolyte (1 M LiPF6 in EC/DMC (3:7))//LiCoPO4 full-cell were analyzed. The highest reversible capacity of MnCo2O4 and LiCoPO4 electrodes was 1167 and 107 mAh g−1, respectively, at 0.1 C-rate. The combination of these two electrodes produced a battery with an operating voltage of 4.2 V and a reversible capacity of 95 mAh g−1 at 0.1C, leading to an energy density of 415 Wh kg−1 based on the cathode mass. These results indicate that the MnCo2O4 is a promising anode for high-performance lithium-ion batteries.

Comprehensive study of a versatile polyol synthesis approach for cathode materials for Li-ion batteries

This work reports a comprehensive study of a novel polyol method that can successfully synthesize layered LiNi0.4Mn0.4Co0.2O2, spinel LiNi0.5Mn1.5O4, and olivine LiCoPO4 cathode materials. When properly designed, polyol method offers many advantages such as low cost, ease of use, and proven scalability for industrial applications. Most importantly, the unique properties of polyol solvent allow for greater morphology control as shown by all the resulting materials exhibiting monodispersed nanoparticles morphology. This morphology contributes to improved lithium ion transport due to short diffusion lengths. Polyol-synthesized LiNi0.4Mn0.4Co0.2O2 delivers a reversible capacity of 101 and 82 mAh·g−1 using high current rate of 5C and 10C, respectively. It also displays surprisingly high surface structure stability after charge-discharge processes. Each step of the reaction was investigated to understand the underlying polyol synthesis mechanism. A combination of in situ and ex situ studies reveal the structural and chemical transformation of Ni-Co alloy nanocrystals overwrapped by a Mn- and Li-embedded organic matrix to a series of intermediate phases, and then eventually to the desired layered oxide phase with a homogeneous distribution of Ni, Co, and Mn. We envisage that this type of analysis will promote the development of optimized synthesis protocols by establishing links between experimental factors and important structural and chemical properties of the desired product. The insights can open a new direction of research to synthesize high-performance intercalation compounds by allowing unprecedented control of intermediate phases using experimental parameters.

Investigation of the Li–Co antisite exchange in Fe-substituted LiCoPO4 cathode for high-voltage lithium ion batteries

Carbon coated olivine Pnma LiCoPO4 (LCP/C) and Fe-substituted LiCo0.8Fe0.2PO4 (LCFP/C) were synthesized by a solvothermal method and their structural features and electrochemical properties were investigated. The electrochemical performance of LCFP/C is better than that of LCP/C, owing to the partial substitution of Co by Fe which effectively suppresses the increasement of antisite exchange between Li+ and Co2+ ions within the structure during cycling, despite a similar amount of Li–Co antisite exchange in pristine LCP/C and LCFP/C samples. Furthermore, direct visualization of Co in Li sites in the pristine samples and after 50 cycles was achieved through high-resolution scanning transmission electron microscopy for both LCP/C and LCFP/C. It was found that LCP/C locally formed a new cation-ordered structure after cycling due to the Li–Co antisite exchange, while the structure of LCFP/C remains almost the same. This study provides direct evidence that Fe substitution reduced the Li–Co exchange and improved the electrochemical cycling life of the LiCoPO4 cathode for high-voltage lithium ion batteries.

Effect of porous structure of LiCoPO4 on its performance in hybrid supercapacitor

Hybrid supercapacitors (HSCs) based on the mechanochemically prepared LiCoPO4 samples with different specific surface area and porosity as cathodes and the activated carbon (AC) Kuraray K-50 as anode were fabricated and studied. The effect of porosity and specific surface area of LiCoPO4 on its performance in HSC was investigated. The porosity was measured within the widest possible range (compared to other methods) of pore size (1–105 nm) using standard contact porosimetry (MSCP). The LiCoPO4 samples, prepared from different precursors, varied in a number of meso- and macropores and the specific surface area (13.9 m2 g−1 and 9.3 m2 g−1), while the specific surface area of AC anode was equal to 1340 m2 g−1. The capacity of the LiCoPO4 cathode with a larger specific surface area and amount of mezopores was superior. To achieve the highest specific power of the LiCoPO4/AC HSC, the active mass ratio of anode and cathode, the electrolyte composition, and the operating voltage range were optimized. As a result, the specific power of 2300 W kg−1 was achieved, which exceeds that observed earlier in the literature.

Investigation of Lithium-Ion Diffusion in LiCoPO4 Cathode Material by Molecular Dynamics Simulation

One of the common cathode materials in the lithium ion battery is the olivine structure LiMPO4, where M is one of Co, Mn, Ni, Fe elements or their combination. Due to its high energy density LiCoPO4 is considered as a cathode material in the lithium ion battery. Lithium ion diffusion at the atomic scale is very important for determining the electrode charge/discharge rate-capability. A molecular dynamics simulation method can be used to investigate the lithium ion diffusion in a material from the atomic point of view. In this study, the diffusevity and structural properties of the LiCoPO4 cathode material are investigated by evaluating the mean square displacement curves, radial distribution function plots, and z-density profiles obtained using the molecular dynamics simulation implemented in the DL-POLY software. The results 10−12 m2/s to 10−13 m2/s at different show that the diffusion coefficient of crystalline LiCoPO4 ranges from 10−2 m2/s to 10−2 m/s at different temperatures. By comparing the diffusion coefficient in different directions, it is found that the motion of lithium ions along the [010] channel is significantly more convenient than that along [100] and [001] channels. By substituting other metals, such as iron, nickel and manganese, for cobalt, the transport and structural properties of the resulting material are investigated. The results indicate that the cobalt-containing structure has a more capability for fast charging and discharging.

Bulk-type all-solid-state batteries with mechanically prepared LiCoPO4 composite cathodes

All-solid-state Li batteries are suitable for the application of 5 V-class cathode materials that can provide high energy densities. However, it is hard to fabricate bulk-type batteries composed of a mixture of active materials and solid electrolytes because of the difficulty of ensuring a conductive path and suppressing the formation of reactive layers at the interface. Herein, we report a simple mechanical processing route to composite cathode granules synthesized from a 5 V-class LiCoPO4 cathode, conductive carbon, and phosphate solid electrolyte. The homogeneously dispersed component particles were granulated during the mechanical treatment. The first discharge capacity is 82 mAh/g for the bulk-type all-solid-state battery obtained by using a composite cathode pellet, polymer solid electrolyte, and Li anode. Scanning transmission electron microscope-electron energy loss spectroscopy measurements did not reveal any Co diffusion or the formation of a resistive layer at the interface between LiCoPO4 and the solid electrolyte. The fabrication of a composite cathode by mechanical treatment is essential for the development of high-energy bulk-type all-solid-state batteries.

Investigation of lithium-ion diffusion in LiCoPO4 cathode material by molecular dynamics simulation

One of the common cathode materials in the lithium ion battery is the olivine structure LiMPO4, where M is one of Co, Mn, Ni, Fe elements or their combination. Due to its high energy density LiCoPO4 is considered as a cathode material in the lithium ion battery. Lithium ion diffusion at the atomic scale is very important for determining the electrode charge/discharge rate-capability. A molecular dynamics simulation method can be used to investigate the lithium ion diffusion in a material from the atomic point of view. In this study, the diffusevity and structural properties of the LiCoPO4 cathode material are investigated by evaluating the mean square displacement curves, radial distribution function plots, and z-density profiles obtained using the molecular dynamics simulation implemented in the DL-POLY software. The results show that the diffusion coefficient of crystalline LiCoPO4 ranges from 10–12 m2/s to 10–13 m2/s at different temperatures. By comparing the diffusion coefficient in different directions, it is found that the motion of lithium ions along the [010] channel is significantly more convenient than that along [100] and [001] channels. By substituting other metals, such as iron, nickel and manganese, for cobalt, the transport and structural properties of the resulting material are investigated. The results indicate that the cobalt-containing structure has a more capability for fast charging and discharging.

Highly [010]-oriented self-assembled LiCoPO4/C nanoflakes as high-performance cathode for lithium ion batteries

In this article, highly [010]-oriented self-assembled LiCoPO4/C nanoflakes were prepared through simple and facile solution-phase strategies at low temperature and ambient pressure. The formation of 5-hydroxylmethylfurfural and levoglucosan via the dehydration of glucose during the reaction played a key role in mediating the morphology and structure of the resulting products. LiCoPO4 highly oriented along the (010)-facets exposed Li+ ion transport channels, facilitating ultrafast lithium ion transportation. In turn, the unique assembled mesoporous structure and the flake-like morphology of the prepared products benefit lithium ion batteries constructed using two-dimensional (2D) LiCoPO4/C nanoflakes self-assembles as cathodes and commercial Li4Ti5O12 as anodes. The tested batteries provide high capacities of 154.6 mA·h·g−1 at 0.1 C (based on the LiCoPO4 weight of 1 C = 167 mA·h·g−1) and stable cycling with 93.1% capacity retention after 100 cycles, which is outstanding compared to other recently developed LiCoPO4 cathodes.

Positive effects of a particular type of microwave-assisted methodology on the electrochemical properties of olivine LiMPO4 (M = Fe, Co and Ni) cathode materials

Pure LiMPO4 (M=Fe, Co and Ni) cathode materials have been synthesized with an ordered olivine structure using a concurrent solvothermal process involving low-level and long-time microwave applications using ethylene glycol as a reaction medium. The influences of this unusual microwave approach have been investigated in detail in terms of the structural and electrochemical properties of the materials produced, and the findings have been compared with those obtained by conventional methods. The Rietveld refinement reveals that all powder products have the same orthorhombic olivine structure in the Pnma space group, that lithium ions are predominantly occupied at the octahedral M1 [100] site, and that the Fe, Co and Ni transition metal-ions reside at the octahedral M2 [010] site. It has also been found that anti-site disorders remain somewhat limited, albeit slightly higher in the case of the Li–Ni couple. LiFePO4 exhibits good long-term electrochemical properties. LiCoPO4 and LiNiPO4 cathodes exhibit qualitative cyclic stability performances in their own categories. At the end of 500 cycles, LiFePO4 retained 97.6% of the initial capacity value, while LiCoPO4 was able to maintain only 52.8% at the end of the 50th cycle. In particular, this extraordinary microwave technique mediates stable and low-defect crystal formation, which has a remarkable effect on the electrochemical stability characteristics of the material. Thus, this work provides a positive contribution for advanced energy storage applications.

Cr-substituted LiCoPO4 core with a conductive carbon layer towards high-voltage lithium-ion batteries

Electrical and ionic conductivity are two major limiting factors for LiCoPO4 cathode material. To overcome these shortcomings, a Cr-substituted LiCoPO4 core with a conductive carbon layer cathode material is synthesized using the sol-gel method. The physical chemistry properties of these materials are systematically investigated by using various characterization methods. For instance, the XRD and Rietveld refinement results reveal that Cr successfully substitutes the Co within the LiCoPO4 core to form LiCo1-1.5xCrxPO4/C (x = 0, 0.02, 0.04, 0.06) without changing the olivine structure but exhibits a decrease in the unit cell volume with increasing Cr substitution. SEM and TEM images indicate that Cr substitution does not lead to changes in the basic morphology of LiCo1-1.5xCrxPO4/C (x = 0, 0.02, 0.04, 0.06) material, which is composed of agglomerated nanoparticles with an 8nm carbon layer on the surface. The EDS and XPS results confirm that Cr is uniformly distributed on the surface and that the oxidation state of Cr is +3. FTIR spectra indicate that the antisite defect concentration decreases with increasing Cr substitution. Furthermore, Cr substitution significantly improves the electrochemical performances of LiCo1-1.5xCrxPO4/C (x = 0.02, 0.04, 0.06) cathode. Notably, the LiCo0.94Cr0.04PO4/C delivers an initial discharge capacity of 144mAhg−1 at 0.1C and shows a capacity retention of 71% after 100 cycles between 3.0 and 5.0V. The CV and EIS results indicate that the polarization is reduced and that the electronic and ionic conductivities are improved by Cr substitution. The good electrochemical performances for Cr-substituted LiCoPO4/C electrodes are attributed to the lower antisite defect concentration, as the reduction of polarization, the improvement of electronic and ion conductivity and the uniform carbon layer. These features will accelerate the commercial application of LiCoPO4 towards the start-art of the high voltage lithium-ion batteries.